In a variety of marine environments, seismic surveys can be taken to gain a better understanding of geological formations beneath a body of water. Relatively large marine regions can be surveyed by a surface vessel or vessels towing seismic streamer cables through the water. Another vessel, or the same vessel, can be employed in providing a seismic source, such as a compressed air gun utilized to generate acoustic pulses in the water. The seismic source is used to generate energy that propagates down through the water and into the geological formation. Marine survey data on the geological formation can be obtained by detecting the energy reflected from interfaces between geological formations. Hydrophones are connected along the seismic streamer cables to detect the reflected energy.
Accurate collection of data by the hydrophones is affected by changes in characteristics of the water column, such as changes in sound velocity between regions of the water column in the survey area. The travel time of the reflected energy/signal through the water column is needed to accurately establish, for example, the depth of the target reflecting surface. In some applications, sound velocity probes are dropped with varying frequency from a survey vessel during the seismic survey to collect data on sound velocity. The usefulness and frequency of the drops, however, can be limited by several factors, including operational safety considerations, risk of tangling the sound velocity probe line with the seismic spread equipment, requirements of the survey client, type of survey, e.g. 4D versus 3D, and knowledge of the survey space and time rate of sound velocity variation.
Current methods to measure sound velocity in the water column during a seismic survey operation include the use of both retrievable and expendable probes. The probes can be deployed either by conventional drop systems or advanced continuous drop systems. However, the approaches can be relatively expensive, cumbersome, and limited in adaptability.